This invention relates to a duo two leading type drum brake device, hereinafter referred to simply as a drum brake device, with a built-in parking brake mechanism. More specifically, this invention relates to a drum brake device that is suitable for use as a rear brake of a commercial automotive type vehicle.
A conventional drum brake device is disclosed in Japanese Patent Publication No. 61-14371. Referring to FIG. 13, Japanese Patent Publication No. 61-14371 illustrates a parking brake operation mechanism in which a pair of brake shoes function as leading shoes for braking operation regardless of the rotational direction of a brake drum.
In the conventional parking brake mechanism, an upper end 3a of a first lever 3 is pivotally supported at an upper side 1a of a first brake shoe 1 and a lower end 4a of a second lever 4 is pivotally supported at a lower side 2b of a second, or other, brake shoe 2. An upper strut 5 is provided between a central region 3b of the first lever 3 and an upper side 2a of the second brake shoe 2. A lower strut 6 is provided between a central region 4b of the second lever 4 and a lower side 1b of the first brake shoe 1. A central region 7b of a brake lever 7 is pivotally supported at a lower end 3c of the first lever 3 at a pivot point 9. A connecting means 8 connects an upper end 4c of the second lever 4 and an upper end 7a of the brake lever 7. Also, a lower end 7c of the brake lever 7 is pulled toward the second lever 4 side to activate operation of the parking brake.
The structure of the above-described conventional parking brake has numerous drawbacks. For example, the conventional parking brake requires additional parts, such as, for example, the first lever 3, the second lever 4, and the connecting means 8 besides the upper strut 5, the lower strut 6, and the brake lever 7. The requirement of the above-listed additional parts makes the structure of the parking brake device more complex, as well as increases the costs associated with manufacturing.
Furthermore, because the components of the conventional parking brake are arranged in the central region of the brake, the conventional parking brake may easily be redesigned for a large diameter brake such as is found in a heavy duty vehicle. However, redesigning the conventional parking brake for a small diameter brake such as is found in a medium duty vehicle is difficult to accomplish because the brake lacks sufficient space for mounting a parking brake mechanism in the central region of the drum brake. Because of the problem of providing sufficient space to mount all of the necessary devices, a duo two leading type drum brake with a built in conventional parking brake mechanism is limited to use for a large diameter brake and is rendered impracticable for a small diameter brake.
Also, the conventional parking brake does not use an automatic parking stroke adjustment mechanism. Thus, a parking brake stroke increases as a lining wears. In addition, the limited space available in the parking brake renders any attempt of providing an automatic parking stroke adjustment mechanism in one of the upper or lower struts 5, 6 undesirable.
Moreover, because the connecting means 8 is positioned on an operation path of the parking brake mechanism and three levers, specifically, the first and second levers 3, 4 and brake lever 7, are pivotally supported by a pin (not identified by a reference number) assembling and taking apart the conventional parking brake become extremely troublesome.
An object of this invention is to overcome the drawbacks of the above-described conventional parking brake.
Another object of this invention is to provide a duo two leading type drum brake device with a parking brake mechanism having a simple structure and lower cost.
Yet another object of this invention is to provide a duo two leading type drum brake device having an automatic parking stroke adjustment mechanism that provides the driver with improved operational feeling.
A first aspect of this invention is a duo two leading type drum brake device comprising a back plate, a pair of service brake actuators, and a pair of brake shoes. The brake shoes face each other on the back plate and each pair of adjacent ends has one of the service brake actuators in between, wherein a pivot lever is rotatably supported longitudinally at a central region of one of the brake shoes. One strut is provided between one end of the pivot lever and the other brake shoe, while another strut is positioned to be parallel to the one strut and is provided between the other ends of the pivot lever and the other brake shoe.
A parking brake actuator comprises a brake lever and, a link mechanism includes said pivot lever and a pair of said struts. Because the parking brake actuator is provided so as to activate said link mechanism, the brake device with a built-in parking brake mechanism of this invention can be designed for use for a small diameter brake.
The parking brake actuator, upon operation of the parking brake, spreads open the pair of brake shoes through an engagement with the link mechanism. An automatic shoe clearance adjustment device, which extends itself in response to an excessive opening of each brake shoe upon operation of service brake and automatically adjusts a clearance between a brake drum and each brake shoe, is provided in each service brake actuator. Also, an automatic parking stroke adjustment device, which extends itself in response to an excessive opening of both brake shoes and automatically adjusts an amount of parking brake actuator stroke, is provided on at least one of the struts.
A second aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein each adjustment convergence value of the automatic shoe clearance adjustment and automatic parking stroke adjustment devices are set to support each adjacent end of both brake shoes by an anchor of the service brake actuator when the service brake is not being operated.
A third aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the automatic shoe clearance adjustment device has a built-in screw mechanism having a bolt and a nut and is housed in a cylinder of the service brake actuator.
A fourth aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the screw mechanism has a reversible screw threaded connection where the bolt and nut are both rotatable and operation of the automatic shoe clearance adjustment device features an exponential incremental characteristic or curve.
A fifth aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the automatic shoe clearance adjustment device uses a ratchet mechanism having a retrofit lever mounted outside of the cylinder body of the service brake actuator and a gear.
A sixth aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein operation by the automatic shoe clearance adjustment device of the ratchet mechanism of the lever and the gear, features a linear incremental characteristic function.
A seventh aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the automatic parking stroke adjustment device has a linear incremental adjusting function providing a screw mechanism of a bolt and a nut.
An eighth aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the automatic parking stroke adjustment device has a one-shot adjusting function providing teeth formed on a bell crank lever and elements that engage the teeth.
A ninth aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the parking brake actuator activates parallel to the back plate.
A tenth aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the parking brake lever operates at a right angle relative to the back plate.
An eleventh aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein a protuberance is integrally formed by pressing longitudinally on a central region of the pivot lever or the shoe web. The protuberance is rotatably pivotally supported in an associated hole provided either on one of either the brake shoe or the pivot lever (without the protuberance).
A twelfth aspect of this invention is to provide a duo two leading type drum brake device as described above, wherein the protuberance on the central region of the pivot lever or the brake shoe is hollow and formed as a burr with a press, wherein a brake shoe freely holds the pin piercing the hollow protuberance.
Accordingly, the number of parts necessary for the parking brake mechanism of this invention is reduced, thereby simplifying the structure and providing a lightweight device at a lower cost. Because the parts of this invention are not concentrated in the brake center, the automatic parking stroke adjustment device may easily be arranged in the brake. Further, the parking brake stroke does not vary even if the lining wears out, which provides the driver a comfortable feeling while operating the parking brake.
By setting the amount of adjustment of the automatic shoe clearance adjustment and automatic parking stroke adjustment devices so that the fixed anchor of the service brake actuator supports each adjacent end of both brake shoes when not in operation, the configuration of this invention prevents dragging or biting of the brake shoes while driving. The automatic shoe clearance adjustment device may have either incremental or one shot adjusting capabilities. Thus, the invention provides a wide range of applications and makes it possible to commonly use the parts in different kinds of brake devices.
Furthermore, this invention provides a parking brake device that is easy to assemble and disassemble.